1. Field of the Invention
The present invention relates to a recording and/or reproducing apparatus (hereinafter, referred to as recording/reproducing apparatus) of an optical recording medium, and particularly to a recording/reproducing apparatus having a controller for a focus servo, a tracking servo and an aberration correction.
2. Description of the Related Art
Optical discs such as a CD (Compact Disc) and a DVD (Digital Video Disc or Digital Versatile Disc) are well known as information recording media for optically recording and reproducing information. Furthermore, there are various types of optical discs, for example, an optical disc for reproduction only, a write-once optical disc on which only additional recording can be done, and a rewritable optical disc on which information can be erased and re-recorded. Research and development are in progress for developing high-density optical discs and an optical pickup device and an information recording and/or reproducing apparatus applicable to the high-density optical discs.
It is considered to irradiate a light beam having a smaller irradiating diameter to the optical disc by increasing the numerical aperture (NA) of an objective lens provided in the optical pickup device so as to cope with the increase in recording density of the optical disc. Further, the light beam having a shorter wavelength is used to cope with the recording density increase.
However, aberration of the light beam caused by an optical disc is increased as the numerical aperture NA of the objective lens is increased or a light beam having a shorter wavelength is used. This makes it difficult to improve performance accuracy of the recording/reproduction of information.
For example, when an objective lens having a large numerical aperture is used, the amount of birefringence distribution, which depends on the incidence angle, is increased at the pupil surface of the optical disc, since the range of the incidence angle of the light beam to the optical disc is increased. This creates a problem that an aberration due to the birefringence becomes more influential. In addition, the adverse effect of aberration increases due to a thickness error of a disc cover layer for protecting the disc recording surface.
Such an aberration correction apparatus is disclosed, for example, in Japanese Patent Laid-Open Publication Kokai No. 2000-131603. The apparatus includes two groups of lenses (or beam expander) to correct spherical aberration of the light beam produced at the optical disc by varying a distance between the lenses along an optical axis of the light beam. Another aberration correction apparatus disclosed in Japanese Patent Laid-Open Publication Kokai No. H10-106012. The apparatus includes a beam expander for varying the beam diameter of the light beam. The beam expander is moved along the optical axis of the light beam to correct the spherical aberration.
As another conventional technique for reducing the aforementioned aberration, an optical pick-up device having a liquid crystal element for aberration correction is proposed. An example of such aberration correction element is disclosed in Japanese Patent Laid-Open Publication Kokai No. H10-269611. The aberration correction element includes a plurality of phase adjustment portions formed concentrically. The phase adjustment portions are applied with predetermined voltages to adjust orientation of the liquid crystal molecules. Thus, the aberration of the light beam can be corrected.
It is necessary to optimize a focusing adjustment and an aberration correction to perform a preferable recording or reproducing operation. The focusing adjustment and the aberration correction are generally performed by detecting a focus error and the aberration of the light beam by using a reading signal (RF signal) from the optical disc. The adjustment and the correction are performed in accordance with the amounts of the focus error and the aberration. An example of such focus control is disclosed in Japanese Patent Laid-Open Publication Kokai No. H8-30975.
However, when the high NA objective lens is used as mentioned above and the aberration correction element is operated so as to optimize the aberration correction, a focus adjusting value is deviated from an optimum value. Otherwise, when the focus adjusting value is controlled so as to be optimum, the aberration correction is deviated from an optimum value. Accordingly, there is a problem that it is difficult to optimize both the focus control and the aberration correction. In other words, there is a problem that the optimization of an entire system cannot be achieved by the individual adjustment or correction of the parameter values.